If you run this script you'll see that it is a
programmer's dream for text based menus. You'll probably notice
that it's very similar to the 'for' construction, only rather
than looping for each 'word' in $OPTIONS, it prompts the user.

What this script does should be clear to you. The expression
in the first conditional tests if the program has received an argument
($1) and quits if it didn't, showing the user a little usage message.
The rest of the script should be clear at this point.